API: renaming const and import

[yorkie]

As @ry mentioned:

"import" and "const" are keywords in JS, so I think "graph.import" and "graph.const" are undesirable choices.

Now a propose is that:

• use constant instead of const by keeping consistence with Python
• use load or other name not in keywords list should address this issue basically

Hi @ry this looks good to you?

[ry]

Yes, those are good alternatives.